This invention relates to methods for testing viscoelastic materials to determine their rheological properties, and to apparatus for performing such testing. More particularly, this invention relates to methods for obtaining values of the storage modulus, the loss modulus, or both, for viscoelastic materials, and to apparatus for obtaining such values.
The response of viscoelastic materials like rubber to deformation is not simple compared to that of a metal spring or a liquid. Rubber has a response somewhere between that of a metal spring and a liquid such as water. A metal spring resists deformation proportionately to the amount of deformation or strain put on the spring. It doesn't matter how slow or fast one moves the spring but only how far it is moved. A liquid such as water resists deformation proportionately to the rate of deformation or strain rate. If one stops moving in water, resistance by the water stops.
When rubber is deformed, some of its resistance to deformation is proportional to the amount of deformation and some of its resistance is proportional to the rate of deformation. The constant used for a particular rubber to describe the magnitude of that rubber's resistance to the amount of deformation is G' or S' (called G Prime or S Prime). Note that G has a scientific meaning which is the shear modulus while S is a general term referring to the "stiffness" of the rubber. A stiffer rubber will have a larger G' or S'. This constant also indicates the amount of energy which can be stored by the rubber. The constant used to describe that same rubber's resistance to the rate of deformation is G" or S" (called G double prime or S double prime). This constant also indicates the amount of energy converted to heat and lost. (The ratio of these two (G"/G' or S"/S') is an important rubber property called Tan (.delta.)).
Prior art devices for testing viscoelastic materials are typified by curemeters or rheometers designed to measure the properties of a rubber compound as it is vulcanized. Such devices often features an oscillating rotor placed in a sample confined under pressure, wherein the rotor is driven by an eccentric mechanism to excite the rubber sample in a sinusoidal pattern and the applied torque is measured. Other devices of this sort eliminate the rotor, and drive one of the confining dies, measuring resultant torque on the opposite die. Typically, values for torque at maximum displacement are obtained, and are assumed to represent the storage modulus of the material; the torque at zero deformation is representative of the loss modulus. Alternatively, after determining torque at maximum displacement, the maximum torque value is also measured, and the value of the loss modulus is calculated by using the pythagorean theorum (assumed to be equal to the square root of the sum of the maximum torque value squared less the square of the value of the torque at maximum displacement). The nature of these calculations is such that errors are "built into" them. A need exists for greater accuracy in calculating the modulus components of viscoelastic materials, so that their behavior is better characterized.